Bug#928132: general: Black background behind some XFCE4 panel plugins

Package: general
Severity: minor
Dear Maintainer,
Behind the Whisker Menu, Power Manager Plugin, PulseAudio Plugin,
and likely some others, there is a black background. I am using all
of the latest packages as of 28 April, 2019. It is likely black
because of my theme, Materia-Dark-Compact. However, I have tested
some other themes, such as the ones that came with XFCE4. In some
light themes, the background is a solid white. This is annoying when
the panel isn't exactly black or white, or if it's transparent, like
mine.
-- System Information:
Debian Release: buster/sid
APT prefers testing
APT policy: (500, 'testing')
Architecture: amd64 (x86_64)
Kernel: Linux 4.19.0-4-amd64 (SMP w/4 CPU cores)
Kernel taint flags: TAINT_PROPRIETARY_MODULE, TAINT_OOT_MODULE, TAINT_UNSIGNED_MODULE
Locale: LANG=en_US.UTF-8, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8), LANGUAGE=en_US.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /usr/bin/dash
Init: systemd (via /run/systemd/system)
LSM: AppArmor: enabled